1. Field of the Invention
The present invention relates to an optical integrated semiconductor device, and more particularly to an optical integrated semiconductor device which is fabricated by way of vapor-phase epitaxial growth such as metal organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE).
2. Description of the Related Art
Optical technology finds more and more applications for meeting requirements for higher operation speeds and larger data-handling capabilities in communications systems for data transmission and exchange and also in information processing systems. Research and development efforts are being made to fabricate various optical devices for use in such applications. The advance in the vapor phase epitaxial growth process has enabled the fabrication of multi-quantum well devices, surface-emitting laser diodes, optical integrated semiconductor devices and ultra high-speed optical semiconductor devices capable of handling data at a rate in excess of 2 Gb/s.
Known ultra high-speed optical semiconductor devices include distributed feedback laser diodes (DFB LD) and distributed Bragg reflector diodes (DBR LD). Of particular interest among these known ultra high-speed optical semiconductor devices is a .lambda./4-shift DFB LD in which light is shifted in phase by .pi./2 at the center of the device so as to enable stable single-longitudinal-mode oscillation at the Bragg wavelength even under high-speed direct modulation. Such a phase-shift structure diffraction grating may be fabricated by photolithography or may be replicated from a mechanically engraved master. However, highly sophisticated techniques and know-how are required to fabricate those devices. It has been reported by M. Okai in WM3, Optical Fiber Communication Conference (OFC) held in 1991 in the U.S., that the spatial hole-burning which is problematic in ordinary phase-shift DFB LDs is improved in a corrugation pitch- modulated (CPM) DFB LD.
Generally, it is possible to provide diffraction gratings with a variety of complex wavefront conversion functions. Therefore, diffraction gratings can be integrated by combining such complex wavefront conversion functions. The diffraction gratings have special shapes including circular, curved, and irregularly periodic shapes. Heretofore, fabricating a partial diffraction grating for integration has required complex and sophisticated processes such as computer-controlled electron beam processes for drawing the required patterns.
In recent years, the rapid development of thin-film crystal epitaxial growth processes including MOVPE and MBE has permitted the fabrication of a semiconductor hetero-interface which has a distinct composition changes as accurate as the thickness of an atomic monolayer. Potential wells and superlattice structures formed by such a hereto-interface have peculiar optical and electric characteristics arising from the wave function of electrons. Many research and development efforts are directed to the application of such potential wells and superlattice structures to actual devices. For example, O. Kayset has reported in detail selective growth using a mask of SiO.sub.2 in The Journal of Crystal Growth, 107 (1991), pp. 989-998. An optical integrated modulator DFB LD based on the application of a selective growth mechanism has been reported by T. Karo, et al. in Web7-1 in European Conference on Optical Communication (ECOC '91).
Sophisticated fabrication techniques are required to fabricate the diffraction gratings of periodically modulated configurations or special shapes including curved and irregularly periodic shapes, and processes of fabricating these devices are highly complex. Accordingly, it has been very difficult to fabricate optical integrated semiconductor devices which incorporate complex diffraction gratings and optical semiconductor devices.